The invention relates to a tablet for use in a coordinate digitizer, and more particularly, to a capacitively coupled tablet in which a matrix comprising two groups of spaced conductors disposed along the coordinate axes is divided into segments, each operable as an independent "key" region and marked with a corresponding character.
An electronic equipment is well known which converts the locus of a pen moving, as when handwriting a picture, into a series of digital coded signals in time sequence. U.S. Pat. No. 3,567,859 (Ellis) issued Mar. 2, 1971 and U.S. Pat. No. 3,732,369 (Cotter) issued May 8, 1973 teach the use of a tablet having a lattice formed by parallel wires running along the coordinate axes and which are spaced apart with a regular interval. In such a tablet, one group of conductors is sequentially pulsed, followed by the pulsing of the other group of conductors. The pen which is used to write a figure on the surface of the tablet has its tip end formed of a conductor for capacitive coupling with wires within the lattice. A signal which is produced on the pen from the pulsed conductor is used to digitalize the coordinates of the pen position on the tablet. The pen position along one coordinate is digitalized, followed by the digitalization of the pen position along the other coordinate. A conventional tablet is designed for inputting a handwritten figure into a computor, and to this end, its surface defines a completely flat plane so as to permit a free movement of the pen thereacross either directly or with a paper interposed.
Recently, there has been a proposal that the surface of the tablet be divided into segments which are marked with specific characters so as to permit the tablet to be used as a keyboard. With such a tablet, each segment represents one key region, which operates to form a digital signal representing a coordinate position. Here, there is no conversion of the locus of the pen into a series of digital signals, but the position of the pen moving from one segment to another in a random manner may be independently digitalized, so that the lattice may comprise two groups of broad or wide conductors in distinction to the earlier tablet having a finely divided lattice so as to improve the resolution of the locus. Since it is intended that each segment is marked with a particular character, there must be a certain surface area in order to provide such indication. However, it is necessary that the lattice points be arranged in a manner such that the pen can pick up a same signal wherever it is placed within a segment.
A keyboard formed by a tablet mentioned above has no limitation on the number of segments or keys, and therefore is preferred for use for inputting into a computor information selected from a great number of items such as Japanese or Chinese letters, for example, or for maintaining an inventory of parts or medicines which are present in as many as several thousand items. However, the proposed tablet which is to be used as a keyboard comprises a flat character sheet disposed on a tablet body which has a flat surface in the same manner as a figure inputting tablet. This results in disadvantages that when the pen is placed against the character sheet, the pen may be displaced from the intended key or may be situated intermediate two adjacent keys, thereby causing an erroneous input. On the other hand, the signal produced on the pen will become maximum when that conductor which is located closest to its tip is pulsed, but a capacitive coupling with conductors of the same group which are located on the opposite sides of the first mentioned conductor also produces signals of gradually decreasing amplitude. Thus, there is a need that this type of tablet be used with a sensor which is only responsive to an output in a pulse train which has the maximum amplitude. Where the keys are relatively closely spaced, the difference in amplitude between the pulse having the maximum amplitude and adjacent pulses will be reduced, rendering it difficult to operate the sensor in an optimal manner. It is essential to completely eliminate the likelihood that the sensor may operate in response to pulses from adjacent keys when the pen is accurately located on an intended key.